The ability to maintain a selected track in flight is a desirable feature in an aircraft. An aircraft travelling along a given ground track from point A to point B may be subjected to in-flight crosswinds which can blow the aircraft off track unless corrections are made by the flight crew. The corrections typically require the aircraft to be headed into the wind to the extent necessary to maintain the aircraft along the desired ground track. An angle between the actual track of the aircraft and true north or magnetic north is known as the true track angle or magnetic track angle, respectively. An aircraft flying along a straight line ground track will maintain a constant track angle.
Conventional autopilot control systems include provisions for holding an aircraft at a fixed track angle. The flight crew select the numerical value of the desired track angle at an autopilot mode control panel. Certain aircraft such as the Boeing 737 also feature an autopilot having a conventional automatic track angle hold system. In this system when control wheel steering (CWS) is selected, wheel force inputs by the pilot command aircraft roll rate. When the pilot removes all wheel force and the airplane is close to a wings level attitude, track angle hold is automatically engaged. A disadvantage of this conventional CWS control is that the pilot must remove any applied force (essentially take his hands off the control wheel) in order to engage track angle hold. This may be impossible if, for example, the pilot is attempting to hold a pitch attitude. In addition, during certain asymmetric or dynamic flight conditions, zero wheel force may not correspond to zero wheel position (ailerons neutral). For example, during forward slip flight the wheel position must be nonzero because the ailerons need to be deflected even for zero roll rate flight. This is a disadvantage because the pilot sometimes will need to release wheel force when the wheel is not centered in order to engage track angle hold.
Other conventional control systems have been disclosed. For example, U.S. Pat. No. 3,222,013 by Perkins discloses an autopilot system which maintains an aircraft track defined by a radio beam.
U.S. Pat. No 3,361,392 by Doniger et al discloses a control system which performs a crosswind "decrab" maneuver to automatically align an aircraft with a runway prior to landing flare.
The ability to maintain a desired track also is a desirable feature while the aircraft is on the ground. For example, during takeoff roll and landing rollout the desired track normally is along the runway centerline. A crosswind may cause the aircraft to deviate from the runway centerline unless the flight crew make appropriate corrective control inputs. Other environmental conditions such as runway ice can aggravate the crosswind problem by increasing the tendency of the aircraft to deviate from the runway centerline.
Conventional autopilot systems have provided landing rollout control and takeoff flight director guidance. However, in conventional systems when the pilot uses the rudder pedals for on-ground directional control, rudder and nosewheel steering deflections are commanded rather than track or heading rates. This can place a large workload on the pilot when controlling aircraft direction during adverse conditions such as during crosswinds, icy runway, or engine failure conditions.